High-Speed Circuit-Switched Data (HSCSD), is an enhancement technlogy of Circuit Switched Data (CSD), the original data transmission mechanism of the GSM mobile phone system. As with CSD, channel allocation is done in circuit switched mode. The difference comes from the ability to use different coding methods and even multiple time slots to increase data throughput.
GSM Circuit Switched Data supports one user per channel per time slot. High Speed Circuit Switched Data (HSCSD) gives a single user simultaneous access to multiple channels (up to four) at the same time. As such, there is a direct trade-off between greater speed and the associated cost from using more radio resources- it is expensive for end users to pay for multiple simultaneous calls.
Assuming a standard Circuit Switched Data transmission rate of 14.4 kilobits per second (kbps), using four timeslots with High Speed Circuit Switched Data (HSCSD) allows theoretical speeds of up to 57.6 kbps. This is broadly equivalent to providing the same transmission rate as that available over one ISDN B-Channel. Some Mobile Switching Centres (MSCs) are limited to 64 kbps maximum throughput- this restriction is removed with GPRS.
In networks where HSCSD is deployed, GPRS may only be assigned third priority, after voice as number one priority and HSCSD as number two. In theory, HSCSD can be preempted by voice calls- such that HSCSD calls can be reduced to one channel if voice calls are seeking to occupy these channels. HSCSD does not disrupt voice service availability, but it does affect GPRS. Even given preemption, it is difficult to see how HSCSD can be deployed in busy networks and still confer an agreeable user experience- i.e. continuously high data rate. HSCSD is therefore more likely to be deployed in start up networks or those with plenty of spare capacity- since it is relatively inexpensive to deploy and can turn some spare channels into revenue streams. High Speed Circuit Switched Data (HSCSD) is however easier to implement in mobile networks than General Packet Radio Service (GPRS) because some GSM vendor solutions require only a software upgrade of base stations and no new hardware. This is not the case with D-AMPS networks and some GSM vendor solutions.
The first innovation in HSCSD was to allow different error correction methods to be used for data transfer. The error correction used in GSM is designed to work at the limits of coverage and in the worst case that GSM will handle. This means that a large part of the GSM transmission capacity is taken up with error correction codes. HSCSD provides several levels of possible error correction which can be deployed according to the quality of the radio link. This means that in the best conditions 14.4 kbit/s can be put through a time slot which, under CSD, would normally only carry 9.6 kbit/s.
The second innovation in the HSCSD radio interface was the possibility to use multiple time slots at the same time. This allows an increase in maximum transfer rates (using four time slots) up to 57.6 kbit/s and, even in the bad radio conditions where the highest level of error correction has to be used, will still lead to a four times speed increase over CSD.
HSCSD require the time slots being used to be fully reserved for a single user. It is possible that either at the beginning of the call, or at some point during a call, it will not be possible for the user's full request to be satisfied since the network is often configured so that normal voice calls take precedence over additional time slots for HSCSD users. The user is then charged, often at a rate higher than a normal phone call, and sometimes multiplied by the number of time slots allocated, based on the period of time that the user has a connection active. This makes HSCSD relatively expensive in many GSM networks and so, packet-switched GPRS which typically has lower pricing is becoming more common than HSCSD connections.